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From: Rich Webb
Subject: Re: digital voltmeter and ammeter
Organization: Line Eater Memorial Fund
X-Newsreader: Forte Agent 1.93/32.576 English (American)
Date: Sun, 19 Jan 2003 15:55:17 GMT
NNTP-Posting-Date: Sun, 19 Jan 2003 10:55:17 EST
On Sun, 19 Jan 2003 14:36:14 +0000 (UTC), Alex Graham
>I have built a NiMH charger, and would like a voltage and current
>readout so the user can keep an eye on the charge, and so that you get
>some feedback from the current setting switch.
>Can anyone suggest a suitable ADC for this? There seem to be hundreds
>available and I have no idea which one to go for. Once I have the ADC,
>what kind of circuitry do I need, and is any of it common to the
>voltmeter and ammeter? (I need a range of about 0-25V and 0-2A)
What about a microcontroller with an ADC bank (which is typically a
single ADC core that's selected to one of several input pins)? Easy
enough to hook it up to an LCD display of the commonly available
Hitachi-type. It would also be possible to have it store results of the
charge or report via it a serial line to a PC.
For a recommendation here, the Atmel AVR series would be a good match
for this project. Program them in-circuit with inexpensive (or home
brew) programmers. Very comfortable to use with assembly language.
There's a GCC port available. The Imagecraft ICC-AVR C compiler is a
good commecial compiler and has a 30 day free trial period. For info and
pointers see www.avrfreaks.com.
>Is it possible to make an ammeter with an ADC in a similar way?
There are more or less complete black-box single chip solutions
available (frequently Hall effect or precision shunt). For a one off,
home brew, solution try an op amp measuring the drop along one of the
charger leads to the battery pack. Scale the op amp gain to get good
resolution on the ADC and calibrate it (as well as you can) with your
regular DMM. It would probably be more repeatable to measure the drop
across a shunt, though.
>Any suggestions would be gratefully recieved! :) however I dont want to
>do this by plugging in a convenient voltmeter chip if possible - Id
>rather make it from a lower level,
Also recommend incorporating temperature monitoring. Thermistors are
simple to use, although non-linear so you'll need to work out the
conversion from ADC counts to temperature. Lots of other temperature
sensor options, too.
Rich Webb Norfolk, VA
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